Watch that horse! (6)
I believe the answer is:
hunter
This clue is a double definition.
'watch' is the first definition.
(type of watch)
'that horse' is the second definition.
(I've seen this before)
(Other definitions for hunter that I've seen before include "Pocket watch; horse" , "One chasing - type of watch" , "someone on safari" , "One after quarry" , "trapper" .)